How do you replace the heater core on a 86 GMC Sierra Classic?

To replace the heater core on a 1986 GMC Sierra Classic, first disconnect the battery and drain the cooling system. Remove the dashboard and the heater assembly by taking out the necessary screws and bolts. Disconnect the heater core hoses from the engine compartment, then remove the old heater core from the heater box. Install the new heater core, reassemble the heater assembly and dashboard, and refill the cooling system before reconnecting the battery.